A thermoplastic resin molded article containing reinforcing fibers such as typically glass fibers or carbon fibers is excellent in mechanical strength, heat resistance and chemical resistance, and is utilized as parts in the field of automobiles, in the field of electric and electronic appliances and in the field of precision machines. However, with the recent requirement for weight reduction thereof, such parts are much down-sized and thinned, and are further required to have increased mechanical strength and dimensional stability; and at the same time, for the molding materials to be used for them, it is required to develop materials capable of satisfying the above-mentioned necessary properties. In particular, in the field of automobiles, the parts to be used in the engine room are required to have sufficient mechanical strength even at high temperatures of 100° C. or more, and resin materials have come to be used for the exterior panels and the structural components for supporting them.
Regarding the reinforcing fiber length in molded articles, it has been known for a long time that the mechanical strength such as rigidity and impact resistance of the articles can be enhanced by prolonging the fiber length. For example, Patent Reference 1 discloses a long fiber-reinforced polyamide resin composition and a molded article produced by injection-molding the resin composition, which are excellent in mechanical strength and moldability and which comprises (A) a polyamide resin, (B) a fibrous reinforcing material having a length of at least 3 mm in an amount of from 5 to 80% by weight (of the composition), and (C) a fatty acid metal salt having from 22 to 32 carbon atoms in an amount of from 0.01 to 3% by weight (of the composition). Patent Reference 2 discloses a resin composition and a molded article produced by injection-molding the resin composition which have a good surface appearance and have a high thermal deformation temperature, and contain a blend of nylon-66/nylon 6=(99 to 1% by weight)/(1 to 99% by weight) as the (A) polyamide resin in Patent Reference 1.
However, the fibrous reinforcing material used in the resin compositions described in Patent References 1 and 2 have a circular cross section because of the easiness in their production, and in case where the fibrous reinforcing material having such a cross-sectional profile is used, even though it may improve the mechanical strength of articles, the fibrous reinforcing material is often ground into powder during injection-molding and the fiber length of the reinforcing material remaining in the molded articles may be shortened, thereby bringing about some problems in that the reinforcing effect of the reinforcing material is lowered, and the molded article is oriented owing to the orientation of the reinforcing material along the resin flow and is thereby warped; and for these reasons, the application of the fibrous reinforcing material of the type is limited to only specific parts. In Patent Reference 2, the fiber length of the fibrous reinforcing material dispersed in the injection-molded article is defined to be at least 1 mm in terms of the weight-average fiber length thereof, but no description is given therein relating to a method of measuring the weight-average fiber length and to the length of the fibers in the articles really formed therein.
Patent Reference 3 describes an invention of approximating the relation between the profile of the fiber length distribution in a long fiber reinforcing material-containing resin molded article, and the moldability and the physical properties of the molded article in accordance with two-parameter Weibull distribution, saying that, when the distribution profile is gentle and tailed from the short side to the long side of the fiber length, then the moldability and the physical properties of the molded article can be well balanced. Patent Reference 4 discloses an exterior molded article of a long fiber-reinforced thermoplastic resin whose anisotropy to be caused by the fiber orientation occurring in injection-molding is reduced, wherein the content of the reinforcing fibers dispersing in the molded article is from 30% by weight to 90% by weight, the weight-average fiber length is from 1.5 mm to 10 mm, the maximum projected area of the molded article is at least 20000 mm2, the flow path length of a narrow flow path having a cross section of at most 100 mm2 in molding is at most 150 mm, the maximum liner expansion coefficient of the part of the molded article having a wall thickness of at least 2 mm is at most 5×10−5 K−1, and the ratio of (maximum linear expansion coefficient)/(minimum linear expansion coefficient) is at most 1.8. However, the reinforcing material used in these techniques has an ordinary circular cross-sectional profile, and the techniques are not still satisfactory in point of the moldability and the impact strength with the fibrous reinforcing material having a long fiber length, the outward appearance of the molded articles, and the dimensional stability thereof free from molding shrinkage and warpage.
To solve these problems, Patent Reference 5 shows that, when the cross-sectional profile of glass fibers, as typical reinforcing fibers, is flattened, then the specific surface area thereof increases as compared with that of glass fibers having a circular cross section and the adhesiveness thereof to a matrix resin composition therefore increases, and when the fiber length in the molded article is prolonged (the average fiber length of the fibers having a circular cross-sectional profile is 0.47 mm, while the average fiber length of the fibers having a cocoon-molded cross-sectional profile is 0.57 mm), then the mechanical strength of the molded article is enhanced. The resin composition described in Patent Reference 5 may be effective for enhancing the tensile strength and the surface smoothness and for preventing the warpage, as compared with the resin composition containing glass fibers having a circular cross section, but is not still satisfactory; and in particular, when a polyamide resin is used as the thermoplastic resin, then the impact strength of the molded article often lowers to the same degree as that in the case where glass fibers having a circular cross section are used.
Patent Reference 6 discloses a method of prolonging the fiber length of the flattened glass fibers so as to solve the problems discussed in Patent Reference 5. Concretely, this describes provision of flattened glass fibers-containing thermoplastic resin pellets, in which a plurality of flattened glass fiber filaments having a flattened cross section are aligned in one direction so that both ends thereof may reach the pellet surface. Patent Reference 6 has a description relating to the length of the fibers remaining in the molded article, but is silent at all on the overall fiber length distribution and on the relationship between the fiber length distribution and the physical properties of the molded article. When the technique is employed, the impact strength of the molded article could be enhanced in some degree; however, since the length of the glass fibers remaining in the molded article is not sufficiently long (in the examples, the length is at most 0.49 mm), the technique could not still satisfactorily solve the problems of mechanical strength depression, strength retentiveness in high-temperature atmospheres, and dimensional change owing to resin shrinkage and warpage.
Patent Reference 1: JP-A 5-9380
Patent Reference 2: JP-A 6-107944
Patent Reference 3: JP-A 9-286036
Patent Reference 4: JP-A 2006-82275
Patent Reference 5: JP-B 2-60494
Patent Reference 6: JP-A 2006-45390